It's an easy way to introduce fractions, especially since it's common to hear/say it's a quarter passed 2, half passed 5, and a quarter to 9.

Also teaches multiples, since the numbers on the clock represent multiples of 5.

Helps with directions, clockwise is when the hands spin to the right and counter-clockwise to the left. You'd be amazed how many students can't tell their left from right.